Optical thickness measurement of occluded samples by lens-less Fourier transform digital holography, thermal loading, and machine learning.

Thickness measurements of objects, especially transparent and semi-transparent objects, are essential for their characterization and identification. However, in the case of occluded objects, the optical thickness determination becomes difficult, and an indirect way must be devised. Thermal loading of the objects changes their opto-thermal properties, which will be reflected as a change in their optical thickness. The key to quantifying such occluded objects lies in collecting these opto-thermal signatures. This could be achieved by imaging the changes occurring to a probe wavefront passing through the object while it is being thermally loaded. Digital holographic interferometry is an ideal tool for observing phase changes, as it can be used to compare wavefronts recorded at different instances of time. Lens-less Fourier transform digital holographic imaging provides the phase information from a single Fourier transform of the recorded hologram and can be used to quantify occluded phase objects. Here we describe a technique for the measurement of change in optical thickness of thermally loaded occluded phase samples using lens-less Fourier transform digital holography and machine learning. The advantage of the proposed technique is that it is a single shot, lens-less imaging modality for quasi-real-time quantification of phase samples behind thin occlusions.

Z-scan model for Laguerre-Gaussian excitation in mode-mismatched thermal lens spectrometry.

In this work, we have introduced a Z-scan thermal lens (TL) model based on Laguerre-Gaussian (LG) L G10 laser induced excitation in a mode-mismatched dual-beam configuration. The analytical expression of the TL signal and its dependence on sample to detector distance as well as the Z-scan have been derived. The theoretical analysis shows that the phase shift and TL signal are higher than the values obtained using an excitation with the T E M 00 Gaussian profile. The experimental demonstration of the theoretical approach has been performed using the L G10 and T E M 00 Gaussian beams, respectively. Experimental proofs of the model are presented and found to be in agreement, demonstrating that Laguerre-Gaussian induced excitation is more sensitive than the Gaussian one.

Measuring the linear optical absorption coefficient by interferometry and the thermal lensing effect: a numerical analysis.

We report on a pump-probe thermal lensing method for measuring the linear absorption coefficient of liquids by using interferometry and numerical analysis. The method is based on interferograms generated when a localized photothermal effect is induced in the sample. The photothermal effect itself is induced by a pump beam impinging on a sample located on-axis of the probe beam, which is one of the paths of a Mach-Zehnder interferometer. A digital camera is employed as the acquisition device allowing the capture and storage of the experimental data. During the experiment, a total of three photographs are taken and stored on a personal computer, and by using an algorithm, the numerical analysis is done. Numerical analysis is subsequently used to calculate the photothermal phase difference and the normalized spatial distribution of the pump beam irradiance. Plotting the phase difference as a function of the spatial distribution of the pump beam produces a linear dependence from which the linear absorption coefficient is obtained. The sensitivity of the method (λ/1500) is validated using ethanol, methanol, and carbon disulfide.

DFG-based mid-IR tunable source with 0.5 mJ energy and a 30 pm linewidth.

We report on a laser system based on difference frequency generation (DFG) to produce tunable, narrow-linewidth (<30pm), and comparatively high-energy mid-IR radiation in the 6.8 µm region. The system exploits a lithium thioindate (LiInS2) nonlinear crystal and nanosecond pulses generated by single-frequency Nd:YAG and Cr:forsterite lasers at 1064 and 1262 nm, respectively. Two experimental configurations are used: in the first one, single-pass, the mid-IR energy achieved is 205 µJ. Additional increments, up to 540 µJ, are obtained by performing double-pass through the nonlinear crystal. This laser has been developed for high-resolution photon-hungry spectroscopy in the mid-IR.

Speckle pattern analysis of crumpled papers.

In this paper, we show that laser speckle analysis (LSA) can provide valuable information about the structure of crumpled thin sheets. Crumpling and folding of slender objects are present in several phenomena and in various ranges of size, e.g., paper compaction, cortical folding in brains, DNA packing in viral capsids, and flower buds, to name a few. The analysis of laser speckles, both numerical and graphical, is a source of information about the activity of biological or non-biological materials, and the development of digital electronics, which brought the ease of image processing, has opened new perspectives for a spectrum of LSA applications. LSA is applied on randomly crumpled and one-, two-, and three-times folded papers, and appreciable differences in LSA parameters are observed. The methodology can be applied for easy-to-implement quantitative assessment of similar phenomena and samples.

Digital imaging information technology for biospeckle activity assessment relative to bacteria and parasites.

This paper reports on the biospeckle processing of biological activity using a visualization scheme based upon the digital imaging information technology. Activity relative to bacterial growth in agar plates and to parasites affected by a drug is monitored via the speckle patterns generated by a coherent source incident on the microorganisms. We present experimental results to demonstrate the potential application of this methodology for following the activity in time. The digital imaging information technology is an alternative visualization enabling the study of speckle dynamics, which is correlated to the activity of bacteria and parasites. In this method, the changes in Red-Green-Blue (RGB) color component density are considered as markers of the growth of bacteria and parasites motility in presence of a drug. The RGB data was used to generate a two-dimensional surface plot allowing an analysis of color distribution on the speckle images. The proposed visualization is compared to the outcomes of the generalized differences and the temporal difference. A quantification of the activity is performed using a parameterization of the temporal difference method. The adopted digital image processing technique has been found suitable to monitor motility and morphological changes in the bacterial population over time and to detect and distinguish a short term drug action on parasites.

Imaging functional blood vessels by the laser speckle imaging (LSI) technique using Q-statistics of the generalized differences algorithm.

In this work, we report about q statistics concept to improve the performance of generalized differences algorithm based on intensity histogram for imaging functional blood vessel structures in a rodent window chamber of a mice. The method uses the dynamic speckle signals obtained by transilluminating the rodent window chamber to create activity maps of vasculatures. The proposed method of generalized differences with q statistics (GDq) is very sensitive to the values of defined parameters such as: camera exposure time, the q value and the camera frame number. Appropriate choice of q values enhances the visibility (contrast) of functional blood vessels but at the same time without sacrificing the spatial resolution, which is of utmost importance for in-vivo vascular imaging.

Online fast Biospeckle monitoring of drug action in Trypanosoma cruzi parasites by motion history image.

This paper reports on the application of the motion history image (MHI) method on dynamic laser speckle processing as a result of a specific drug action on Trypanosoma cruzi parasites. The MHI procedure is based on human action recognition, and unlike other methods which use a sequence consisting of several frames for recognition, this method uses only an MHI per action sequence for recognition. MHI method avoids the complexity as well as the large computation in sequence matching-based methods and detects a change in the speckle pattern. Experimental results of MHI on real-time monitoring of activity (motility) under the influence of the drug demonstrate the effectiveness of the proposed method. The MHI showed an online result without loss of resolution and definition if we compare with routine LASCA method. The obtained results highlight the advantage of the MHI analysis over traditional qualitative image intensity-based methods and demonstrate the potential of measuring the activity of parasites via dynamic laser speckle analysis. The data was further numerically analyzed in the time domain, and the results presented the ability of the technique to monitor the action of the drug, particularly Epirubicin (100 µg/ml).

Gold Nanoparticles Modulate Excimer and Exciplex Dynamics of PDDCP-Conjugated Polymers.

How plasmonic nanostructures modulate the behavior of exciplexes and excimers within materials remains unclear. Thus, advanced knowledge is essential to bridge this gap for the development of optoelectronic devices that leverage the interplay between plasmonic and conjugated polymer hybrid materials. Herein, this work aims to explore the role of gold nanoparticles (AuNPs) in modulating exciplex and excimer states within the conjugated polymer poly(2,5-di(3,7-dimethyloctyloxy) cyanoterephthalylidene) (PDDCP), known for its photoluminescent and semi-conductive properties, aiming to create innovative composite materials with tailored optical features. The spectral analysis was conducted to investigate the effects of AuNPs on the PDDCP, varying AuNP volume percentages to measure changes in the absorption profile, molar extinction coefficient (ε), absorption cross-section (σa), and optical bandgap (Eg). Fluorescence spectra of the mixture at different volume ratios were also examined to assess exciplex formation, while amplified spontaneous emission (ASE) profiles were analyzed to study the behavior and photochemical stability of the polymer-NP hybrid material. Increasing AuNP volume induced both blue and red shifts in the absorption profile of the PDDCP. Higher AuNPs concentrations correlated with decreased ε and σa, inversely impacting Eg. The emission spectra obtained at varied AuNP volume ratios indicated significantly enhanced exciplex and excimer formations. The ASE profiles remained consistent but showed reduced intensity with increasing AuNPs concentrations, indicating their influence on hybrid material behavior and stability. The findings suggest that AuNPs affect PDDCP's optical characteristics, altering the absorption profile, bandgap, and fluorescence spectra. Furthermore, the observed reduction in ASE intensity highlights their influence on the behavior and photochemical stability of the hybrid material. These results contribute to a better understanding of the versatile applications of plasmonic-conjugated hybrid polymers.

Fluid-Infiltrated Metalens-Driven Reconfigurable Intelligent Surfaces for Optical Wireless Communications.

A reconfigurable intelligent surface (RIS), a leading-edge technology, represents a new paradigm for adaptive control of electromagnetic waves between a source and a user. While RIS technology has proven effective in manipulating radio frequency waves using passive elements such as diodes and MEMS, its application in the optical domain is challenging. The main difficulty lies in meeting key performance indicators, with the most critical being accurate and self-adjusting positioning. This work presents an alternative RIS design methodology driven by an all-silicon structure and fluid infiltration, to achieve real-time control of focal length toward a designated user, thereby enabling secure data transmission. To validate the concept, both numerical simulations and experimental investigations of the RIS design methodology are conducted to demonstrate the performance of fluid-infiltrated metalens-driven RIS for this application. When combined with different fluids, the resulting ultra-compact RIS exhibits exceptional varifocal abilities, ranging from 0.4 to 0.5 mm, thereby confirming the adaptive tuning capabilities of the design. This may significantly enhance the modulation of optical waves and promote the development of RIS-based applications in wireless communications and secure data-transmission integrated photonic devices.

Fluid-responsive tunable metasurfaces for high-fidelity optical wireless communication.

Optical wireless communication (OWC), with its blazing data transfer speed and unparalleled security, is a futuristic technology for wireless connectivity. Despite the significant advancements in OWC, the realization of tunable devices for on-demand and versatile connectivity still needs to be explored. This presents a considerable limitation in utilizing adaptive technologies to improve signal directivity and optimize data transfer. This study proposes a unique platform that utilizes tunable, fluid-responsive multifunctional metasurfaces offering dynamic and unprecedented control over electromagnetic wave manipulation to enhance the performance of OWC networks. We have achieved real-time, on-demand beam steering with vary-focusing capability by integrating the fabricated metasurfaces with different isotropic fluids. Furthermore, the designed metasurfaces are capable of polarization-based switching of the diffracted light beams to enhance overall productivity. Our research has showcased the potential of fluid-responsive tunable metasurfaces in revolutionizing OWC networks by significantly improving transmission reliability and signal quality through real-time adjustments. The proposed methodology is verified by designing and fabricating an all-dielectric metasurface measuring 500 µm × 500 µm and experimentally investigating its fluid-responsive vary-focal capability. By incorporating fluid-responsive properties into spin-decoupled metasurfaces, we aim to develop advanced high-tech optical devices and systems to simplify beam-steering and improve performance, adaptability, and functionality, making the devices suitable for various practical applications.

Thermal lensing approach based on parabolic approximation and Mach-Zehnder interferometer.

A thermal lensing approach based on parabolic approximation and Mach-Zehnder interferometer for measuring optical absorption and thermal diffusivity coefficients in pure solvents is described in this work. The approach combines the sensitivity of both thermal lensing methods and interferometry techniques. The photothermal effect is induced by a pump laser beam generating localized changes in the refractive index of the sample, which are observed as a shift in phase of the interference pattern. Each interference pattern is recorded by means of a digital camera and stored as digital images as a function of time. The images are then numerically processed to calculate the phase shifting map for a specific time. From each phase shifting map, the experimental phase difference as a function of time is calculated giving a phase-time transient, which is fitted to a mathematical model to estimate the optical absorption and thermal diffusivity of the sample. The experimental results show that the sensitivity is approximately λ/4800 for the minimum phase difference measured.

The phase range extension and accuracy improvement in Fresnel biprism-based digital holography microscopy.

We report a highly stable and affordable dual-wavelength digital holographic microscopy system based on common-path geometry. A Fresnel biprism is used to create an off-axis geometry, and two diode laser sources with different wavelengths λ1 = 532 nm and λ2 = 650 nm generate the dual-wavelength compound hologram. In order to extend the measurement range, the phase distribution is obtained using a synthetic wavelength Λ1 = 2930.5 nm. Furthermore, to improve the system's temporal stability and reduce speckle noise, a shorter wavelength (Λ2 = 292.5 nm) is used. The feasibility of the proposed configuration is validated by the experimental results obtained with Molybdenum trioxide, Paramecium, and red blood cell specimens.

Measuring Thermal Diffusivity of Azoheteroarene Thin Layers by Photothermal Beam Deflection and Photothermal Lens Methods.

Measurement of thermal properties of thin films is challenging. In particular, thermal characterization is very difficult in semi-transparent samples. Here, we use two photothermal methods to obtain information about the thermal diffusivity as well as thermal conductivity of azoheteroarene functionalized polymer thin layers. The photothermal beam deflection (PBD) method is employed to gather data directly on thermal conductivity and thermal diffusivity, while the thermal lens (TL) method is employed to measure the effective thermal diffusivity. Consequently, the thermal diffusivity of the layers is indirectly estimated from the effective thermal diffusivity using a well-established theoretical relationship. Despite the utilization of distinct methods, our study reveals a remarkable consistency in the highly accurate results obtained from both approaches. This remarkable agreement reaffirms the reliability and mutual compatibility of the employed methods, highlighting their shared ability to provide accurate and congruent outcomes.

Thermal Diffusivity and Conductivity of Polyolefins by Thermal Lens Technique.

A mode-mismatched thermal lens spectrometry (TLS) technique, in a pump-probe two-laser-beam configuration, was employed for the experimental determination of the thermal properties of four selected well-characterized polyolefin homopolymer films. We investigated the thermal diffusivity (D) and thermal conductivity (κ) of high-density polyethylene, low-density polyethylene, linear low-density polyethylene, and polypropylene. We also measured the structural properties (i.e., average molecular weight, polydispersity index, branching number), along with the rheological and thermal properties (i.e., melting point, specific heat capacity Cp, degree of crystallinity) of samples by high-temperature gel permeation chromatography (HT-GPC), rheometric mechanical spectrometry (RMS), differential scanning calorimetry (DSC), and densitometry. The relationship between microstructural properties such as degree of crystallinity, D, and κ was investigated. The results show that there is good correlation between the degree of crystallinity and D. The TL technique enables measurement of D in semitransparent thin films within an uncertainty of 4%.

DNA-Directed Protein Anchoring on Oligo/Alkanethiol-Coated Gold Nanoparticles: A Versatile Platform for Biosensing Applications.

Herein, we report on a smart biosensing platform that exploits gold nanoparticles (AuNPs) functionalized through ssDNA self-assembled monolayers (SAM) and the DNA-directed immobilization (DDI) of DNA-protein conjugates; a novel, high-sensitivity optical characterization technique based on a miniaturized gel electrophoresis chip integrated with online thermal lens spectrometry (MGEC-TLS), for the high-sensitivity detection of antigen binding events. Specifically, we characterized the physicochemical properties of 20 nm AuNPs covered with mixed SAMs of thiolated single-stranded DNA and bio-repellent molecules, referred to as top-terminated oligo-ethylene glycol (TOEG6), demonstrating high colloidal stability, optimal binder surface density, and proper hybridization capacity. Further, to explore the design in the frame of cancer-associated antigen detection, complementary ssDNA fragments conjugated with a nanobody, called C8, were loaded on the particles and employed to detect the presence of the HER2-ECD antigen in liquid. At variance with conventional surface plasmon resonance detection, MGEC-TLS characterization confirmed the capability of the assay to titrate the HER2-ECD antigen down to concentrations of 440 ng/mL. The high versatility of the directed protein-DNA conjugates immobilization through DNA hybridization on plasmonic scaffolds and coupled with the high sensitivity of the MGEC-TLS detection qualifies the proposed assay as a potential, easily operated biosensing strategy for the fast and label-free detection of disease-relevant antigens.

Synthesis and Characterization of MWCNT-COOH/Fe3O4 and CNT-COOH/Fe3O4/NiO Nanocomposites: Assessment of Adsorption and Photocatalytic Performance.

In this study the adsorption and photodegradation capabilities of modified multi-walled carbon nanotubes (MWCNTs), using tartrazine as a model pollutant, is demonstrated. MWCNT-COOH/Fe3O4 and MWCNT-COOH/Fe3O4/NiO nanocomposites were prepared by precipitation of metal oxides in the presence of MWCNTs. Their properties were examined by X-ray diffraction in powder (XRD), Fourier-transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, synchrotron-based Scanning PhotoElectron Microscopy (SPEM), and Brunauer-Emmett-Teller (BET) analysis. It was found that the optimal adsorption conditions were pH 4 for MWCNT-COOH/Fe3O4 and pH 3 for MWCNT-COOH/Fe3O4/NiO, temperature 25 °C, adsorbent dose 1 g L-1, initial concentration of tartrazine 5 mg L-1 for MWCNT-COOH/Fe3O4 and 10 mg L-1 for MWCNT-COOH/Fe3O4/NiO and contact time 5 min for MWCNT-COOH/Fe3O4/NiO and 15 min for MWCNT-COOH/Fe3O4. Moreover, the predominant degradation process was elucidated simultaneously, with and without simulated sunlight irradiation, using thermal lens spectrometry (TLS) and UV-Vis absorption spectrophotometry. The results indicated the prevalence of the photodegradation mechanism over adsorption from the beginning of the degradation process.

Online electrophoretic nanoanalysis using miniaturized gel electrophoresis and thermal lens microscopy detection.

Online thermal lens microscopy (TLM) coupled with gel electrophoresis (GE) can represent a powerful tool for separating and detecting a wide range of biomaterials. Unlike slab gel electrophoresis (SGE), the proposed method does not require prolonged procedure between separation and detection. In this work, we developed an online monitoring GE system to separate and detect nanosized materials. The design is based on a homemade and cost-effective miniaturized GE chip (MGEC) integrated with real-time TLM detection through microcontroller-based digitization board platform. To validate the feasibility and practicability of the proposed approach, we evaluated its separation capability via employing synthesized Fe3O4-Au core-shell nanoparticles (NPs) which served remarkably for the proof-of-concept. The optimum conditions for the separation process were achieved through optimization of the excitation power as 30 mW, detection position at 24 mm, the concentration of agarose gel 0.5 % w/v, and 37.5 V/cm as the effective electric field strength. The findings showed that two populations of Fe3O4-Au, core-shell, and uncapped Fe3O4 NPs, were effectively separated in less than eleven minutes, demonstrating rapid assessment of the nanomaterial production quality. Moreover, other characterization techniques such as HRTEM and EDX were employed to confirm the presence of the two dissimilar kinds of NPs separated using MGEC-TLM. The sensitivity of the method was demonstrated by determining the limit of detection (23 pM) for 10 nm AuNPs. It is envisaged that our presented system enables rapid, economical, low volume of reagents consumption and high potential analysis for quality test in various bioanalytical and nanotechnological applications.

Through-Plane and In-Plane Thermal Diffusivity Determination of Graphene Nanoplatelets by Photothermal Beam Deflection Spectrometry.

In this work, in-plane and through-plane thermal diffusivities and conductivities of a freestanding sheet of graphene nanoplatelets are determined using photothermal beam deflection spectrometry. Two experimental methods were employed in order to observe the effect of load pressures on the thermal diffusivity and conductivity of the materials. The in-plane thermal diffusivity was determined by the use of a slope method supported by a new theoretical model, whereas the through-plane thermal diffusivity was determined by a frequency scan method in which the obtained data were processed with a specifically developed least-squares data processing algorithm. On the basis of the determined values, the in-plane and through-plane thermal conductivities and their dependences on the values of thermal diffusivity were found. The results show a significant difference in the character of thermal parameter dependence between the two methods. In the case of the in-plane configuration of the experimental setup, the thermal conductivity decreases with the increase in thermal diffusivity, whereas with the through-plane variant, the thermal conductivity increases with an increase in thermal diffusivity for the whole range of the loading pressure used. This behavior is due to the dependence of heat propagation on changes introduced in the graphene nano-platelets structure by compression.

Photodegradation mechanisms of reactive blue 19 dye under UV and simulated solar light irradiation.

In this work we performed dye photodegradation experiments in presence of TiO2 and Cu/Zr modified TiO2. The changes in the shape of the spectra of RB19 caused by photocatalysts under the simulated solar or UV light were monitored. Since the predominant photocatalytic mechanism can only be observed in very dilute solution of RB19, UV-Vis absorption spectrometry for higher concentrations and thermal lens spectrometry for lower concentrations have been applied to elucidate the mechanistic details of degradation processes. Bleaching of the dye was a characteristic feature, that occurred under both simulated solar and UV lights. It was also evident, that the absorption peak with maximum centered at 592 nm undergoes a slight blue shift during irradiation. The experiments carried out using UV and simulated solar light demonstrated, that two different processes responsible for the RB19 dye degradation occurred. In the initial stage of irradiation one of the processes appears under the UV light and can be recognized by a characteristic blue shift in the absorption spectrum of the solution. The second process is characteristic for irradiation by the simulated solar light which involve a blue shift at longer periods (100 min). These phenomena were attributed to the photocatalytic and photosensitization mechanisms, respectively. However, photocatalytic mechanism was also observed under simulated solar radiation, when the initial dye concentration was decreased to 5 mgL-1, and was recognized by the increase of the thermal lens signal during the initial stages of degradation process. This was possible because the thermal lens spectroscopy technique provides a limit of quantification for RB19 at the concentration level of 0.12 mg L-1, while UV-Vis spectrometry enables quantification of RB19 only down to 4 mg L-1 levels.